Cruise control no workie

[JCampbell]

The cruise control recently stopped working on my ’88 Suburban. Worked fine one day, stopped working the next. Anyone have any hints or tips on where to start? Perhaps someone knows the more common failure points.

I’ve got a 2,000 mile round trip at the end of the month and I’d really like to have a functioning cruise system.

[piecesparts]

Did you look at the brake/vacuum switch? If it is not engaging properly, the control will never take over. I just changed out the controller module on my 84, due to intermittent operation. It is just above the gas pedal on my truck.

[JCampbell]

I haven't looked at anything yet. We took a 400 mile trip this weekend and my wife let me know it wasn't working as we were backing out of the driveway. :/

Anyway to test the switch? I'm not into part swapping as a means of diagnosis.

[piecesparts]

I am going to ask you a couple questions first. Does your cruise have a big servo module on the intake manifold that has vacuum hoses running to it or is it the design that has a electronic control box on the engine bay side of the firewall in front of the driver's seat (by the brake booster)? If it is the latter, then everything is controlled by your computer speed circuit and I can't help you to much. Have you ever been under your dash board and looked at the wiring harnesses? If you have, then look at the brake pedal and follow it up towards the point where it pivots. About halfway up you should find the brake light switch and if you have the first style cruise setup, then there should be a another plunger tyoe switch there that looks like the brake light switch. There will be a vacuum hose connected to it. This is your cruise kill switch when you push on the brake. It is mounted in a springlike nut contraption that allows the switch to actuate when it is no longer against the metal piece that pushes the plunger in, thus breaking the vacuum hold on the cruise module. This switch may have slipped in the springnut and is not contacting the metal piece solidly enough to keep the cruise form operating. All you may have to do is reach up to that switch and with the brake pedal slightly depressed, push the switch towards the metal tab and make sure it contacts the piece solidly when the brake pedal is not being pushed in.

Parts for the first style cruise is getting hard to find, but if you search the internet, there are sources for the control box and servo in a rebuilt forum. I just happened to have an extra control box in my tool chest and I replaced it without any other issues.

[JCampbell]

I figured it out. The linkage between the cc diaphragm and the throttle body had come off somehow.